The present invention relates to an anticalcification treatment for aldehyde tanned biological-tissue, and more particularly to such a treatment which minimizes in vivo calcification and reduces cytotoxicity without impairing the mechanical strength of the tanned tissue.
When biological tissue containing the amino groups (NH.sub.2) characteristic of protein are treated with glutaraldehyde (CHO(CH.sub.2).sub.3 CHO), the glutaraldehyde links the amino-containing proteins together in cross-linking reactions. One of the resulting cross-linked products is devoid of any free aldehyde groups because both aldehyde groups of the glutaraldehyde react with different amino groups, but the other resulting cross-linked product retains a free aldehyde group because both amino groups react with the same aldehyde group of the glutaraldehyde. As both types of cross-linked products are present in equilibrium with each other, the glutaraldehyde-tanned biological tissue will always contain free aldehyde groups.
The free aldehyde groups in an implant are associated with the induction of a local inflammatory reaction and subsequent calcification of the graft. While vigorous washing of the aldehyde-treated tissue is effective to remove unreacted glutaraldehyde, it is ineffective to remove the free aldehyde groups of the glutaraldehyde which has reacted with the tissue protein. The known treatments (e.g., amino acids in an acidic buffer) of the aldehyde-tanned biological tissue to remove the free aldehyde groups from the glutaraldehyde which has reacted with the tissue protein result in an impairment of the mechanical strength of the tanned tissue, thereby frustrating the very purpose of the tanning treatment.
Bioprosthetic heart valves in current use around the world are made or porcine aortic valves or bovine pericardium tanned with glutaraldehyde. Biological arterial substitutes are made of xenograft or homograft vessels treated with glutaraldehyde treated bovine pericardium. Artificial heart valves made from glutaraldehyde treated tissue are excellent substitutes for diseased natural valves, with a particularly low risk of thromboembolism. Their utility is, however, compromised by the aforementioned tendency to calcification which occurs in almost 100% of children within 5 years after implantation. Adults with hyperparathyroidism or renal failure share this tendency. Calcification causes failure of the implants, and the need for re-replacement is a bar to frequent use thereof except in the elderly.
The alternative of bioprosthetic valve replacement using untanned tissue is unacceptable since the untanned tissue undergoes partial digestion and resorption, loses its strength over time and eventually ruptures. Tanning of the tissue with aldehyde minimizes or prevents its loss of strength over time.
The alternative of mechanical valve replacement carries with it the significant disadvantages of an absolute requirement for anticoagulation (to prevent valve thrombosis) and a continuing incidence of emboli, even with proper anticoagulation. These problems are magnified in the Third World, where large numbers of children have valvular heart disease and difficulty with anticoagulation management is accompanied by an excessive rate of both thrombosis and hemorrhage.
A bioprosthetic valve that did not calcify would become the valve of choice, replacing not only all bioprosthetic valves in current use, but being preferred also in a substantial number of those cases where mechanical valves are in current use. Such a valve would make possible an increase in valve replacement surgery and would improve results, especially in developing countries. Tanned biological tissue for other uses has a smaller commercial production, but the behavior and possibilities for improved preparation are similar. To date, however, no successful anticalcification treatment has been described.
Artificial heart valves made from aldehyde tanned tissue have the further disadvantage of a cytotoxicity which inhibits endothelialization of implants made therefrom. While the cytotoxicity presents less of a problem to the use of aldehyde tanned implants than the calcification potential thereof, it remains a problem which impedes the widespread adoption of aldehyde tanned implants, especially where endothelialization of the implant is deemed critical.
Accordingly, it is an object of the present invention to provide a method of treating aldehyde tanned biological tissue to minimize in vivo calcification.
Another object is to provide such a treatment method without diminution of the mechanical strength gained by tanning with aldehydes.
A further object is to provide such a treatment method which also reduces the cytotoxicity of aldehyde tanned implants that would otherwise inhibit endothelialization of implants.
It is also an object of the present invention to provide such a treatment method which is simple, economical and safe.